1. Field of The Invention
This invention relates to a control system for variable displacement hydraulic pumps for use in hydraulically operated construction vehicles such as for example power shovels etc., with a gyratory upper body, and more particularly to such a control system as having three control functions, i.e., flow control for varying the angle of swash plate of the pump in proportion to the manipulating position of respective work implement controlling valve, constant torque control in proportion to the delivery pressures of the variable displacement hydraulic pumps, and cut-off control for reducing relief losses.
2. Description of The Prior Art
As disclosed in the Japanese patent application Laid-open Publication No. Sho 55-1478, the vehicles of the kind specified have so far employed variable displacement hydraulic pumps as hydraulic pumps therefor to achieve effective utilization of the engine output, and most of them have been of the constant torque control type.
If only the constant torque control is employed, however, the control system has been disadvantageous in that (1) the hydraulic pump is located at a maximum swash plate angular position even when the vehicle is not working, and so the total discharge volume (the delivery) is wasted as a pressure loss, (2) when the actuator is inching, a greater part of the discharge volume is returned from the valve at its neutral position into a tank resulting in a loss, and (3) when relieving the pressurized fluid, most of the discharge volume turns into a relief loss.
To eliminate the above-mentioned disadvantages, there has been proposed the Japanese patent application Laid-open Publication No. Sho 55-43245.
The above-mentioned invention is arranged such that a hydraulic servo mechanism is used to control the angle of the swash plate of the pump and such reducing valves as a TCC valve, a CO valve and a NC valve are located in a control pressure circuit which provides input for the hydraulic servo mechanism to detect the delivery pressure of the pump and the flow rate of the fluid through the neutral circuit of the valve to vary the control pressure which provides the input for the hydraulic servo mechanism thereby controlling the angle of the swash plate of the pump.
Further, one of the prior art reducing valve means used for the above-mentioned flow rate controls is disclosed in the specification of the U.S. Pat. No. 3,990,352. According to this reducing valve means, two sets of springs for determining the characteristic of the outlet pressure of the reducing valve means are attached to provide a same loading or compressive force, and therefore when the rod on which the two springs are mounted moves, one of the springs having a lower spring constant commences to deflect in the first place. As a result, only one pattern can always be obtained as the characteristic of the outlet pressure.